(1) Field of the Invention
The present invention relates generally to amplification of laser beams, and more particularly to a system that can generate high-power, coherent, single-frequency and single transverse mode light pulses using a low power laser.
(2) Description of the Prior Art
A variety of laser applications such as remote sensing of the environment require the use of a sequence of high-power, single-frequency, single transverse mode laser pulses. Often, several pulses in a sequence are required to have the same frequencies and transverse mode structures. The same frequency and transverse mode structure is crucial to the performance of systems that involve interferometry between signals that are generated by different pulses.
Current systems attempt to achieve the single transverse mode requirement by design of the generating laser cavity. The control and matching of the frequency is generally achieved by using a low-power, continuous-wave, single-frequency laser to seed the cavity of a Q-switched pulse laser.
The laser cavity design comprises one mirror of the pulsed laser cavity attached to a piezoelectric mount that can be repositioned with the application of a control voltage. A control circuit is used to adjust the piezoelectric mount such that the frequency of the laser pulse is adjusted to be a controllable frequency difference from the seed laser pulse beam. This is achieved by taking advantage of the fact that the build-up time of the laser pulse depends on the difference between the seed laser beam frequency and the nearest piezoelectrically-controlled resonant frequency of the pulsed cavity.
The pulse build-up time is a minimum when the resonant frequency of the pulsed cavity laser is equal to the frequency of the seed laser beam. The control circuit works by altering the position of the piezoelectrically-controlled mirror laser by a small distance to either side of the matched frequency mirror position. Measurements of the pulse build-up time are made for pulses generated with the mirror on both sides of the frequency matching position. The differences in pulse build-up are used to correct any position errors.
When the pulsed laser is activated, a search procedure is used to establish the desired operating point. While the result is the generation of a sequence of high-power light pulses that have frequencies with known variations on either side of the seed laser beam; the alteration rate must be suitable to keep up with the cavity drifts caused by temperature variations and slow drifts in the seed beam frequency.